This invention relates to a pipe laying method and apparatus wherein a pilot pipeline of a small diameter is accurately introduced and laid between a departure pit and an arrival pit and then pipes of intermediate diameters (say, 250-600 mm.phi.) are laid.
In the conventional system, a pilot pipeline is laid underground, by means of a pilot head whose direction can be corrected, between a departure pit and an arrival pit, and subsequently a cutter head (which houses a hydraulic motor etc. taking into account the spaces available in the pits) having a pipe of an intermediate diameter attached thereto is mounted to the rear end of the pilot pipeline and then it is propelled underground by means of a propulsion jack. The earth and sand in front of the pipe of an intermediate diameter are excavated by the action of a cutter mounted on the cutter head so that they may be introduced therein and then conveyed by a screw conveyor mounted in the pilot pipeline to the arrival pit.
Subsequently, the earth and sand which have been transported and discharged will be recovered once into an excavated earth and sand recovery tank and then discharged aboveground.
The above system is described in U.S. Pat. Nos. 4,020,641, 4,024,741 and 4,026,371 and contents therein are incorporated herein by reference.
A problem associated with this system resides in that since the earth and sand are transported through the pilot pipes by the screw conveyor the inside surface of the pilot pipes will wear by abrasion heavily resulting in a reduction of the strength of the pilot pipes which tends to cause buckling of them when they are propelled.
This pipe laying method is further disadvantageous in that the screw conveyor tends to get clogged with the earth and sand so that a required torque cannot be developed thus making the propulsion of pipes to be laid underground impossible.